Systemic and local factors and the maintenance of bone quality.

Bone formation, an essential process for the maintenance of bone mass and strength, depends on changes in osteoblast number or function. Bone formation is modified by systemic hormones such as parathyroid hormone, growth hormone, insulin and steroids, and by local factors that act in an antocrine or paracrine fashion on the osteoblast. Skeletal cells synthesize platelet-derived growth factors and fibroblast growth factors, agents which affect osteoblast cell replication. In addition, skeletal cells synthesize insulin-like growth factors and transforming growth factors beta, agents which also affect the differentiated function of the osteoblast. Systemic and local factors that modify bone formation are likely critical in the maintenance of normal bone.